This invention generally pertains to the field of electronic temperature sensors ("sensors") and configuring such sensors in circuits that compensate for non-linearity of the output of the sensors as a function of temperature.
Sensors of the type described above are typified by the part designated LM335 by the National Semiconductor Corporation and are commercially available. The LM335 is described in National's "Linear Device Data Book" (1994 National Semiconductor Corporation) at pages 1-11 of the section entitled "LM135/LM235/LM335, LM135A/LM235A/LM335A Precision Temperature Sensors" and dated May 1992, which is incorporated by reference herein. (These pages of National's Linear Device Data Book will be referred to as "National's Data Book" below).
(Because of National's part designations, there is actually no part number designated the "LM335". The part available from National having the characteristics of the LM335 in National's Data Book has a part number with "Z" suffix: LM335Z. This suffix simply designates that the part comes in a plastic package. Thus, when referring to the LM335 below, the actual part meant to be referred to from National is the LM335Z.)
The voltage output of typical temperature sensors such as the LM335 are non-linear. The LM335 and like temperature sensors are typically incorporated into circuits that provide for the setting a bias voltage output, but do not compensate for the non-linearity of the sensor.
Thus, a disadvantage of the prior art circuitry was that it did not correct for the non-linearity of the temperature sensor.
The present invention overcomes the deficiencies of the prior art by providing electronic circuitry that corrects the non-linearity of the temperature sensor, such as the non-linearity created by the temperature related drift of the output of the temperature sensor.
According to the invention, the non-linear temperature sensor is configured together with electronic circuitry. A first portion of the electronic circuitry interfaces with the sensor and adjusts the output of the sensor. The first portion of the electronic circuitry, for example, may interface to apply a voltage at a bias of the sensor, thereby adding a constant voltage to the output of the sensor.
Also according to the invention, a second portion of the electronic circuitry is provided. The second portion of the electronic circuitry adjusts the parameters of the first portion of the electronic circuitry over the operating range of the sensor. By adjusting the first portion of the electronic circuitry, the output of the sensor is adjusted.
The second portion of the electronic circuitry is configured so that the adjustment to the output of the sensor caused by the first portion of the electronic circuitry substantially counteracts the non-linear drift of the sensor over the operating range of the sensor.
Where the first portion interfaces with a bias point of the sensor, the second portion may, for example, alter the first portion in a manner that the voltage supplied to the bias point adjusts the voltage output of the sensor to be approximately linear over its operating range.
In a particular embodiment of the invention, the sensor has a bias point that is adjusted by a first portion of electronic circuitry comprising a series connection of a resistor, a potentiometer and a second resistor. The bias point of the sensor is connected to the contact point of the potentiometer. The second portion of the electronic circuitry comprises a thermistor having a temperature coefficient opposite that of the sensor. The thermistor is connected in parallel with one of the resistors of the first portion. The effective resistance of the resistor of the first portion is thus altered by the thermistor of the second portion. By changing the effective resistance, the voltage at the contact point of the potentiometer and thus the bias point of the sensor is changed in a direction opposite that of the drift of the sensor. Selection of the values of the electronic components of the first and second portion can create a change in bias voltage that substantially counteracts the voltage drift of the sensor over the operating range of the sensor, thereby making the sensor output substantially linear.